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Can Zs at DB ever be less than the Zs of the feeding circuit?

Former Community Member
Former Community Member
I am reviewing an EICR recently issued for a building with several Distribution Boards feeding sub-Distribution Boards.

I have noted that in some instances, the figure recorded for 'Zs at this board' is significantly less than the Maximum Measured Zs for the circuit recorded on the feeding DB.

e.g. DB FF4 is recorded as being fed from DB FF1.  The feeding circuit to DB FF4 is recorded as having a Maximum measured Zs of 0.4 Ohm, but the 'Zs at this board' for FF4 is recorded as 0.05 Ohm - which is less than the 'Zs at this board' recorded for FF1 (0.08 Ohm) - and which, is in fact, in turn itself less than the 'Maximum measured Zs' for the circuit feeding it.  Can this be true or are there errors in the report?  I thought that cascaded Zs can only get larger due to the added impedance of the feeding circuits? This is not my primary area of expertise, but I am concerned that the EICR is being used to justify the upgrade of several circuits which have passed previous inspections with no problem (hope the resolution of the extracts from the EICR below are sufficient resolution to read)...

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05733e3016557d58306811936bac5e50-huge-image.png


Many thanks if anyone is able to confirm my concerns or otherwise put me straight...
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    Peter S3:
    @MHRestorations: that is entirely possible- we note that the Cal date for his test meter was 2 days AFTER the date of the inspection...


    I think that annual independant calibration is not the best way if used solely.

    lf A robust frequent checking even with something you built yourself is a good start.

    Before Calcard I built up a resistance box for a few continuity and ins res checks.

    A few sockets for loops and two RCD sockets for RCD tests (2 in case one failed).

    I run all the tests monthly or more frequently.

    I do not claim any great actual accuracy but it makes drifting easier to spot over a short period between such tests and a longer period overall. Not foolproof but has some benefit.

     


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  • 0
    Peter S3:
    @MHRestorations: that is entirely possible- we note that the Cal date for his test meter was 2 days AFTER the date of the inspection...


    I think that annual independant calibration is not the best way if used solely.

    lf A robust frequent checking even with something you built yourself is a good start.

    Before Calcard I built up a resistance box for a few continuity and ins res checks.

    A few sockets for loops and two RCD sockets for RCD tests (2 in case one failed).

    I run all the tests monthly or more frequently.

    I do not claim any great actual accuracy but it makes drifting easier to spot over a short period between such tests and a longer period overall. Not foolproof but has some benefit.

     


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